1. Field of the Invention
The present invention relates to a method of heat-setting fabrics containing bicomponent fibers comprising poly(ethylene terephthalate) and poly(trimethylene terephthalate).
2. Description of Background Art
Fabrics containing poly(ethylene terephthalate) fibers can be heat-set in order to stabilize their dimensions, for example at about 350-360xc2x0 F. (177-182xc2x0 C.), but such fabrics exhibit little or none of the stretch-and-recovery which has become increasingly desirable.
Polyester bicomponent fibers having latent crimp can also be used in making stretch fabrics, for example as described in Japanese Patent JP61-32404 and U.S. Pat. No. 5,874,372. However, not all such bicomponent fibers have adequate stretch-and-recovery properties, and fabrics made from such fibers can also have undesirable characteristics such as poor dye washfastness and uneven surface appearance.
Japanese Published Patent Applications JP58-115144, JP11-189923, and JP05-295634 and Japanese Patent JP63-42021 disclose various processes for treating fabrics comprising bicomponent fibers made from poly(ethylene terephthalate) and other polyesters, copolyesters, or poly(ethylene terephthalate) having a different molecular weight. However, these fibers generally have inadequate crimp, and the methods require excessively high temperatures, additional twisting of the bicomponent fibers, and/or two heat-setting steps, in order to obtain the desired flat but stretchy fabric. Such additional processing of fiber and/or fabric is inefficient and costly, and an improved method of making stretch fabrics comprising polyester bicomponent fibers is needed.
A process for treating a knit fabric comprising a plurality of self-crimping bicomponent fibers comprised of poly(ethylene terephthalate) and poly(trimethylene terephthalate) and having a crimp contraction value (CCa) of at least about 10%, comprising the steps of:
a) stretching the fabric cross-directionally by about 1-15% based on the dry width of the fabric;
b) heat-setting the stretched fabric by dry heat-setting at a temperature of about 160-177xc2x0 C. for a period of about 20-60 seconds or steam heat-setting at a temperature of about 120-145xc2x0 C. for a period of about 3-40 seconds;
c) dyeing the fabric; and
d) drying the fabric without heat-setting it further.
As used herein, xe2x80x9cself-crimpingxe2x80x9d refers to the ability of certain polyester bicomponent fibers spontaneously to form a spiral crimp when drawn, heat-treated and allowed to relax. Additional crimp, beyond that generated by drawing and heat-treating the fiber, can be created during hot-wet finishing of the fabric, for example during dyeing. Such fibers can be knit and woven to create stretch fabrics, for example into tricot, double knit, plain woven, and twill constructions.
As used herein, xe2x80x9cbicomponent fiberxe2x80x9d means a fiber comprising a pair of polymers adhered to each other along the length of the fiber, so that the fiber cross-section is a side-by-side or eccentric sheath-core cross section.
It has now been unexpectedly found that use of a single heat-setting step in a specific temperature range, carried out on woven or knit fabrics comprising certain self-crimping bicomponent fibers, under low cross-direction tension, and before dyeing, results in fabrics having a highly desirable combination of high recovery from stretching (xe2x80x9cunload powerxe2x80x9d), excellent dye washfastness, and a smooth surface appearance and hand.
The polyester bicomponent fibers used to make the fabrics treated by the present process comprise poly(ethylene terephthalate) and poly(trimethylene terephthalate), which can be in a side-by-side or eccentric sheath/core relationship; side-by-side is preferred for maximum crimp development. The weight ratio of the two components is about 70:30 to 30:70. The bicomponent fibers have a crimp contraction value, as hereinafter defined, of at least about 10%. In the fibers, it is preferred that poly(ethylene terephthalate) have a lower intrinsic viscosity (IV) than poly(trimethylene terephthalate). It is not required to twist the bicomponent fibers in order to make the fabrics to be treated, and in fact it is preferred that such a twist not be introduced, since it requires an additional step and therefore generates additional cost.
Optionally, either or both components of the fibers of the fabrics treated by the present process can incorporate comonomers, as long as the beneficial effects of the invention are not adversely affected. For example, the poly(ethylene terephthalate) can incorporate comonomers selected from the group consisting of linear, cyclic, and branched aliphatic dicarboxylic acids having 4-12 carbon atoms (for example, butanedioic acid, pentanedioic acid, hexanedioic acid, dodecanedioic acid, and 1,4-cyclo-hexanedicarboxylic acid); aromatic dicarboxylic acids other than terephthalic acid and having 8-12 carbon atoms (for example isophthalic acid and 2,6-naphthalenedicarboxylic acid); linear, cyclic, and branched aliphatic diols having 3-8 carbon atoms (for example 1,3-propane diol, 1,2-propanediol, 1,4-butanediol, 3-methyl-1,5-pentanediol, 2,2-dimethyl-1,3-propanediol, 2-methyl-1,3-propanediol, and 1,4-cyclohexanediol); and aliphatic and araliphatic ether glycols having 4-10 carbon atoms (for example, hydroquinone bis(2-hydroxyethyl) ether, or a poly(ethyleneether) glycol having a molecular weight below about 460, including diethyleneether glycol). Isophthalic acid, pentanedioic acid, hexanedioic acid, 1,3-propane diol, and 1,4-butanediol are preferred because they are readily commercially available and inexpensive. Isophthalic acid is more preferred because copolyesters derived from it discolor less than copolyesters made with some other comonomers. The comonomer can be present in poly(ethylene terephthalate) at a level of about 0.5-15 mole percent.
Further, the fiber of the invention can contain minor amounts of other comonomers in one or both components, provided such comonomers do not adversely affect the level of fiber crimp or other properties. Such other comonomers include 5-sodium-sulfoisophthalate, at levels of about 0.2-5 mole percent. Very small amounts of trifunctional comonomers, for example trimellitic acid, can be incorporated for viscosity control.
The fabrics to be treated can also include wool, cotton, acetate, rayon, and other suitable fibers along with the polyester bicomponent fibers. Weft (circular and flat bed) and warp knit fabrics can be used in the process of the invention.
In the process of the invention, the heat-setting can be carried out dry or with steam. Dry heat-setting temperatures of about 320-355xc2x0 F. (160-179xc2x0 C.), preferably 165-175xc2x0 C., are used, and the heat-setting time is about 20-60 seconds. Steam setting is carried out at about 248-293xc2x0 F. (120-145xc2x0 C.), preferably 120-130xc2x0 C., for about 3 to 40 seconds. In either case, the longer times can be used at the lower temperatures, and the shorter times can be used at the higher temperatures. During heat-setting, the fabric is kept stretched in the cross-direction by about 1-15%, based on the dry width of the fabric. This is done in order to avoid crepe in the final fabric. By xe2x80x9c1%xe2x80x9d stretch is meant restraint to prevent relaxation during heat-setting; in practice, this means just sufficient tension (stretch) to hold the fabric or apparel on the heat-setting equipment. When the bicomponent fiber has a low crimp contraction value (as defined below), the cross-direction stretch can be low (but within the stated range), and when the fiber has a high crimp contraction value, the stretch can be higher (again within the stated range). The tension (stretch) applied can be used to adjust the finished fabric weight and stretch. Heat setting can be carried out in fabric or apparel form and with any suitable equipment, for example a tenter frame or boarding form.
Dyeing of the heat-set fabric can be conducted with any suitable dye, for example, disperse or acid dyes, by any suitable means, for example beck, paddle, or jet dyeing, and at any temperature appropriate for the particular dye being used.
Drying of the dyed fabric is conducted at sufficiently low temperatures (for example, less than about 145xc2x0 C.) to avoid further heat-setting.
Steam-relaxation before heat-setting can be advantageous to reduce picking and running of the greige fabric when the crimp contraction value of the bicomponent fiber is low, for example, less than 30% as in Examples 3-6. Such steam-relaxation can be performed by any suitable means, for example with a semi-decator, a steam compactor or a tubular steamer.
Tensile properties of the bicomponent fibers were measured according to ASTM D2256 using a 10-inch (25.4-cm) gauge length sample at 65% RH and 70xc2x0 F. at an elongation rate of 60% per minute. Tenacity and initial modulus are reported in deciNewtons per tex, and elongation-at-break as a percentage.
Intrinsic viscosity (IV) of the fiber was measured by exposing polymer to the same process conditions as polymer actually spun into bicomponent fiber except that the test polymer was spun through a sampling spinneret (which did not combine the two polymers into a single fiber) and then collected for IV measurement.
Unless otherwise noted, crimp contraction values in the bicomponent fiber made and used in the Examples were measured as follows. Each sample was formed into a skein of 5000+/xe2x88x925 total denier (5550 dtex) with a skein reel at a tension of about 0.1 gram per denier (gpd) (0.09 dN/tex). The skein was conditioned at 70+/xe2x88x922xc2x0 F. (21+/xe2x88x921xc2x0 C.) and 65+/xe2x88x922% relative humidity for a minimum of 16 hours. The skein was hung substantially vertically from a stand, a 1.5 mg/den (1.35 mg/dtex) weight (e.g. 7.5 grams for a 5550 dtex skein) was hung on the bottom of the skein, the weighted skein was allowed to come to an equilibrium length, and the length of the skein was measured to within 1 mm and recorded as xe2x80x9cCbxe2x80x9d. This 1.35 mg/dtex weight was left on the skein for the duration of the test. Next, a 500-gram weight (100 mg/d; 90mg/dtex) was hung from the bottom of the skein, and the length of the skein was measured to within 1 mm and recorded as xe2x80x9cLbxe2x80x9d. Crimp contraction value (%) (before heat-setting, as described below for this test), xe2x80x9cCCbxe2x80x9d, was calculated according to the formula CCb=100xc3x97(Lbxe2x88x92Cb)/Lb. The 500 g-weight was removed, and the skein was then hung on a rack and heat-set, with the 1.35-mg/dtex weight still in place, in an oven for 5 minutes at about 225xc2x0 F. (107xc2x0 C.), after which the rack and skein were removed from the oven and conditioned as above for two hours. This step was designed to simulate commercial dry heat-setting, which is one way to develop the final crimp in the bicomponent fiber. The length of the skein was measured as above, and its length was recorded as xe2x80x9cCaxe2x80x9d. The 500-gram weight was again hung from the skein, and the skein length was measured as above and recorded as xe2x80x9cLaxe2x80x9d. The after heat-set crimp contraction value, xe2x80x9cCCaxe2x80x9d, was calculated according to the formula CCa=100xc3x97(Laxe2x88x92Ca)/La.
To determine dye washfastness, pieces of the fabrics treated by the process of the invention were given a standard wash stain test (American Association of Textile Chemists and Colorists Test Method 61-1996, xe2x80x9cColor Fastness to Laundering, Home and Commercial: Acceleratedxe2x80x9d; 2A version at 122xc2x0 F. (50xc2x0 C.)), which is intended to simulate five washes at low-to-moderate temperatures. The test was run in the presence of a nylon 6,6 knit fabric, and the degree of staining of the nylon was visually rated.
To determine fabric stretch, three specimens of 3 inxc3x978 in (7.6 cmxc3x9720.3 cm) were cut from the fabric and folded in the middle to form an open loop. The long dimension of each specimen was the dimension tested. The fabric cross-direction (CD) and machine-direction (MD) stretch were tested on separate samples. Unload Power was tested on the cross-direction sample. Each open loop was stitched together about 1 inch (2.5 cm) from its ends to form a closed loop 6 inches (15.2 cm) in circumference. The mechanical properties of the fabric loops were tested with an Instron tensile tester with a 6-inch (15.2 cm) cross head, pneumatic clamps (size 3C, having 1 inxc3x973 in (2.5xc3x977.6 cm) flat faces, 80 psi (552 kPa) air supply), and 10 inches per minute (25.4 cm/min) chart speed. A u-shaped rod was clamped sideways between one of the sets of clamps of the tensile tester so that the ends of the rod [(2.78 in (7 cm) between the ends, 3 in (7.6 cm) around the ends)] projected from the clamps far enough to hold the fabric loop securely. The loop was placed around the projecting rod ends and stretched to a 12-pound (5.4 kg) force and relaxed; the cycle was performed a total of 3 times. xe2x80x9cFabric stretchxe2x80x9d was measured on the 3rd cycle extension at 12-pound (5.4 kg) force, and unload power was measured at 30% remaining available stretch on the 3rd cycle relaxation. xe2x80x9c30% remaining available stretchxe2x80x9d means that the fabric had been relaxed 30% from 12-pound (5.4 kg) force. In order to compare fabrics of different basis weights, unload power was normalized by dividing the as-determined value by the fabric weight.
For Examples 1 and 2, 149 denier (165 dTex), 68 filament bicomponent yarn was prepared by melting poly(trimethylene terephthalate) (60 wt %, IV=1.27 dl/g) and poly(ethylene terephthalate) (40 wt %, IV=0.54 dl/g) in independent melt systems at about 280xc2x0 C., transporting the polymers to a spinneret, and spinning them side-by-side into a cross-flow quench provided with about 100 cfm (2.8 cubic meters per minute) of air. Each component contained 0.3 wt % TiO2. An organic ester-based finish emulsion was applied (5 wt %) to the yarn. The yarn was passed around a feedroll, through a steam draw jet, and then around a second roll to provide a draw ratio of 2.8. The yarn was then passed through a 165xc2x0 C. hot chest containing two rolls to provide a second draw ratio of 1.3. A total of 7.5 wraps were taken between the two rolls in the hot chest. The yarn was passed around a puller roll, through dual interlace jets, and then around a letdown roll. Finish was then reapplied (5 wt %) to the yarn. The yarn was then wound onto a paper core tube. The resulting fibers had a tenacity of 3.5 g/d (3.1 dN/tex), elongation-to-break of 15%, and a crimp contraction (CCa) value of about 46-50%.
In Examples 1 and 2, each fabric was a single jersey knit prepared on a 28 gauge, 24 position circular knitting machine with 255 inches (648 cm) course length per revolution and contained only the bicomponent yarn.